The applicant has developed fuel injection systems for two and four stroke internal combustion engines where the delivery of fuel to the engine is effected by means of compressed air. In large capacity engines, the compressed air can be sourced from an air compressor mounted on or near the engine. The cost of providing a separate air compressor is normally however too high for low cost small capacity engines such as small marine engines, motorbike and scooter engines and lawnmower engines. The requirement for a separate air compressor is therefore avoided in the direct injection system described in the applicant's Australian patent application no. 65608/94 wherein compressed gas from the combustion chamber is trapped and stored within an accumulator which is in communication with a fuel delivery injector. This is achieved by keeping the delivery injector open for a predetermined period after the completion of the injection process when the gas pressure in the combustion chamber is generally above that at the time of injection of the fuel into the combustion chamber of the engine. This allows compressed gas (largely air) from the combustion chamber to travel back through the delivery injector into the accumulator wherein the gas is trapped when the delivery injector closes. The trapped compressed gas is then used to assist in the injection of fuel during the next fuel injection event.
It has however been found in small engines using such a fuel injection system that wear may occur on the valve seat of the delivery injector and the portion of the valve member of the delivery injector contacting the valve seat. Furthermore, combustion and other deposits or build-up may form in and around the fuel exit area of the delivery injector due to the various products of combustion contained in the compressed gas which is trapped for the subsequent fuel delivery event and also due to the proximity of the delivery injector fuel exit area to the combustion chamber. These deposits or build-up are typically "carbonaceous" deposits and may include carbon, partial combustion products and deposits, fuel breakdown deposits, gums and varnishes. These aspects are undesirable in that the overall durability and performance of the injector is typically reduced. In particular, the repeatability of satisfactory spray formation from the delivery injector is reduced.
it is considered that one reason for the occurrence of wear at the contact area between the valve seat and the valve member of the delivery injector is that the fuel and gas exiting the delivery injector provides little to no lubrication of that area such that there is direct metal to metal sliding contact as the valve member closes against the valve seat. More generally, wear is likely to be a problem where the final injector is used to meter low lubricity fuels including liquefied petroleum gas (LPG). Further, there is typically a not insignificant metal to metal impact each time the valve member closes against the valve seat.
It is believed that a similar wear problem can also occur when a gaseous fuel is injected by a fuel injection system. Such a gaseous fuel injection system is described in the applicant's Australian patent application no. PN4895 filed on Aug. 18, 1995, details of which are incorporated herein by reference. The gaseous fuel, more so than in the case of a liquid fuel entrained in a gas, is likely to provide little to no lubricating effect at the contact area between the valve member and valve seat resulting in wear at that area.
Indeed, it is apparent that this wear problem is more serious in fuel injection systems delivering gaseous fuel or liquid fuel entrained in a gas, in single fluid injection systems, the valve member typically closes against a volume or head of incompressible liquid. This acts to provide a degree of dampening of the impact of the valve member against the valve seat. However, where gaseous or predominately gaseous fuel is being delivered, the valve member closes against a volume of compressible fluid. There is therefore less of a dampening or arresting effect than would typically be the case in single fluid liquid systems. Nonetheless, such single fluid injection systems are still likely to be susceptible to the carbon deposit formation problems mentioned herein above. Further, wear may still be a problem in such single fluid injection systems, this wear being a factor of the degree of impact within the system versus the viscosity and compressibility of the fluid being injected. The degree of impact within the system is related to the rate and frequency of closing and/or opening of the injector of single fluid system.
Further, it has been observed by the applicant that significant wear may occur on injector valve surfaces in direct injection two-stroke engines and that the use of detergents/cleaners to control combustion and other deposits may not be effective to prevent injector wear.
Temperature is also known to have some effect on wear and the formation of deposits or build-up. Typically, once temperature has increased to a certain first level, the tendency for deposits or build-up to occur increases as fuel breakdown is encouraged, Further temperature increase beyond this first level generally results in the removal or burning-off of these deposits. However, at these temperatures, the wear resistant properties of the injector material are compromised. Further temperature increases typically further compromise the integrity of the injector material and hence its resistance to wear. Also, as temperature increases, there is typically a corresponding decrease in the hydrodynamic dampening affect provided by the fuel being delivered by the injector. Lubricants such as oil however are typically comparably less effected in regard to this dampening affect by temperature increases.
It is therefore an object of the present invention to avoid at least one of the above problems.